Radio frequency (RF) devices transmit an information signal from one point to another by moving the information signal to a higher frequency range that is more suitable for transmission over the medium being used. This process is known as upconversion. As used herein, “radio frequency signal” means an electrical signal conveying useful information and having a frequency from about 3 kilohertz (kHz) to thousands of gigahertz (GHz), regardless of the medium through which such signal is conveyed. Thus an RF signal may be transmitted through air, free space, coaxial cable, fiber optic cable, etc. An RF transmitter mixes the desired signal, known as the baseband signal, with an RF carrier frequency for transmission over the selected medium. An RF receiver then mixes the signal with the carrier frequency to restore the signal to its original frequency.
In a typical RF receiver the RF signal is received by an antenna, then amplified using a fixed gain amplifier and transmitted over a transmission line having a characteristic impedance to the receiver circuitry. Since the received RF signal strength can vary significantly depending on the distance between the receiver and the transmitter, RF receivers typically include a circuit for automatic gain control (AGC). AGC is helpful to the operation of the receiver because it helps to keep the power level of the received signal close to the optimal level. The signal at the other end of the transmission line is then typically amplified in a variable gain low noise amplifier (LNA) whose gain is controlled by the AGC circuit before being converted from RF to baseband.
One known method of AGC uses a variable resistor T-network. This T-network includes resistors that whose resistances can be made to vary together. The T-network allows the transmission line termination impedance to be kept at the desired value while the signal is properly attenuated, resulting in near optimal power transfer. However resistor T-networks exist as discrete components, which add to system cost. Furthermore resistor T-networks are not very accurate when implemented in conventional, low-cost CMOS integrated circuits. It would be desirable, therefore, to provide new RF circuitry capable of integration but with performance comparable to or better than such known discrete circuits.